Nelson, D. L., & Cox, M. M. (2017). Principles of Biochemistry 7e. W. H. Freeman. 13.3: Phosphoryl Group Transfers and ATP: Inorganic Polyphosphate Is a Potential Phosphoryl Group Donor.
(This part has been removed from the eighth edition of the book.)
PPK-1 and PPK-2 are essentially catalyzing the same reaction, however, PPK-1 is used when polyphosphate synthesis is thermodynamically favorable and PPK-2 is used when the reverse reaction(ATP synthesis) is favorable. What is the need for having 2 different enzymes if 1 enzyme is enough to catalyze the forward and reverse reaction?